1. Field of the Invention
The present invention is in the field of optics, specifically in optical transformers or transformers for bi-directionally coupling optical radiation between two waveguides. It is also in the field of mode transformers between two waveguides.
2. Prior Art
A mode transformer between two different waveguides is an essential part of an optical system where the lightwave (mode) from one optical component is coupled into another component. In optical communication, a mode transformer between an optical fiber waveguide and a high index difference (difference in the refractive indices of core and cladding) planar waveguide is crucial for successful implementation of planar lightwave circuits (PLC) in fiber communication. Therefore, developing an efficient mode transformer between two waveguides has been a subject of intense research.
When transforming the modes between two waveguides with different index (refractive index) differences and/or core indices, high coupling loss arises due to the difference in the mode size, shape, and mode velocity. For example, the index difference and the mode of a fiber optic waveguide are different from those of a high index difference planar waveguide, resulting in a high coupling loss when the fiber optic waveguide and the high index difference planar waveguide are coupled directly. The index difference of a fiber, which is xcx9c0.01, is smaller than that of high index difference waveguides, which is typically equal to or larger than 0.3 in a square channel waveguide configuration, making the fiber mode larger than the high index difference waveguide mode. A channel waveguide is a dielectric waveguide whose core is surrounded by a cladding that is composed of a material or materials with refractive indices lower than that of the core, and wherein the peak optical intensity resides in the core. High index difference waveguides can be defined in other waveguide geometries including a rib waveguide. A rib waveguide is a dielectric waveguide whose core is surrounded by a cladding that is composed of materials of which at least one has the same refractive index as that of the core. In waveguide configurations that are difference from a channel waveguide, a high index difference waveguide is defined as one that has a mode-field size similar to that of a high index difference channel waveguide (within 50% difference in cross-sectional area). In these waveguides, cladding is defined as a region where the evanescent field of optical modes exists.
In addition, the core index of the fiber optic waveguide is lower than that of the high index difference planar waveguide causing a mode velocity difference between two waveguides. When such a change in mode properties takes place too quickly, high power loss arises.
There have been several approaches to achieve efficient mode coupling between two waveguides with different index difference, including mode transformation by tapering the dimension of high index difference waveguide. Mode transformation by a taper has been shown in various publications. Over the tapering region of the high index difference waveguide, the thickness or the width of the waveguide core is gradually tapered down from that of the normal guiding region to a lower thickness or width. As the mode travels from the normal guiding region of the high index difference waveguide into the tapering region, the mode experiences decreasing amount of the core material. The fraction of the mode field distribution that exists outside the core material increases, changing the mode size. The index of the waveguide that the mode experiences is effectively changed by the presence of the taper. In other words, the xe2x80x9ceffective indexxe2x80x9d is gradually changed by the taper. By gradually changing the effective index from that of the low index difference waveguide to that of the high index difference waveguide, the mode coupling can be achieved between two waveguides without high loss. The method to determine the effective index is described in xe2x80x9cThe Handbook of Photonicsxe2x80x9d, Boca Raton, Fla. CRC Press, 532-535 (1996) by M. Gupta.
Mode transformation based on tapering is shown in the prior art including IEEE Photonics Technology Letters, Vol. 5, No. 9, September 1993 by Brenner et al. In this publication, the core of the planar waveguide is vertically tapered down from that of the regular waveguide. The mode size propagating in the tapered region increases due to the reduction of the effective index, and thus the reduction of the effective index difference. This publication shows the gradual mode transformation occurring in one waveguide due to the presence of a taper.
U.S. Pat. No. 5,199,092, issued to Stegmueller et al. shows the coupling of modes between two different waveguides: one broad and one narrow. The two waveguides run parallel to one another and are superimposed with each other to provide a superimposing waveguide guidance. During the superimposed waveguide guidance, one of the two waveguides is tapered down in vertical dimension, while the other waveguide dimension is kept constant. The role of the tapered waveguide is to provide a gradual effective index change, and thus mode transformation, same as the cases in journal publications including that by Brenner et al. The difference is the superimposition of the narrow waveguide in the broad waveguide, providing waveguiding in the broad waveguide once the narrow waveguide is completely terminated by the vertical taper. The broad waveguide is surrounding the narrow waveguide over the whole waveguiding distance. The presence of the broad waveguide helps guiding the mode once the mode transformation is complete.
In addition to single taper devices described above, dual tapers are used in mode transformation between two different waveguides. IEEE Photonics Technology Letters, Vol. 7, No. 5, May 1995 by Zengerle et al., reports a mode transformer having two channel waveguides, each with a taper, one sitting on top of the other. Electronics Letters, Vol 29, No. 4, February 1993 by Schwander et al., reports a mode transformer having two rib waveguides, each with a taper, a portion of one embedded within the other. Both of the rib waveguides used in this art are weakly guiding and thus it is not a suitable method for mode transformation to or from a high index difference waveguide. IEEE Journal of Selected Topics in Quantum Electronics, Vol 3, No 6, December 1997 by Moerman et al. summarizes these dual taper methods for mode transformation.
None of the prior art reports an efficient mode transformer between a low index difference and a high index difference waveguide. This invention discloses, for the first time, an efficient mode transformer based on an embedded dual-taper, useful for transforming the mode to and from a high index difference waveguide.
In accordance with the invention, there is provided an optical mode transformer using an embedded dual-taper, to achieve low-loss mode coupling between two waveguides, one of them having much higher index difference than the other. The transformer can be used to couple an optical mode from an optical fiber, whose typical single-mode dimension of the core is approximately 10 xcexcm in diameter, to the mode in a high index difference planar waveguide, whose single-mode dimension of the core is typically equal to or less than 1 xcexcm in a channel waveguide. The index difference of a square channel waveguide corresponding to a 1 xcexcm single-mode dimension is xcx9c0.3.
It is an objective of the invention to provide a device for transforming the mode between two waveguides with different mode sizes and indices. It is another objective of the invention to provide a device to enable low-loss coupling between the optical fiber waveguide mode and the high index difference planar waveguide mode.
In the invention, the mode undergoes a low-loss transformation between a low index difference waveguide and a high index difference waveguide by traveling through the coupling region containing a dual-taper. The dual-taper provides a gradual change in the mode properties necessary for low-loss, bi-directional mode transformation. Both the low index difference and high index difference waveguides are tapered, in opposite directions. These two oppositely running tapers are placed so that there is an overlap of two waveguides, with the smaller waveguide embedded in the larger waveguide.
It is an objective of the invention to show that the dual-taper structure enhances mode transformation efficiency between two waveguides. It is another objective of the invention to demonstrate the two tapered waveguides should be overlapped, with the overlapping length ranging up from 0, for low-loss mode transformation.
The embedded dual-taper technology disclosed in this invention is suitable for an efficient mode transformation to and from a high index difference waveguide. This has not been possible by the technology shown in prior art, which was limited to a low index difference rib waveguide configuration.
To apply the invention for coupling the modes between an optical fiber and a high index difference waveguide, the low index difference waveguide can be chosen to have similar index difference, core index, and mode size as an optical fiber. The mode from the fiber is initially coupled to the low index difference waveguide having similar properties. Therefore the coupling is achieved with low loss due to the similarity of the modes. Once coupled, light is guided in the low index difference waveguide. Then the mode and the effective index of the low index difference waveguide are gradually changed to those of the final waveguide by the dual-taper.